A multilayer circuit board (MLB) has, among other things, a number of metal layers defining circuit patterns, and a number of insulating layers there-between. The metal layers defining circuit patterns today are typically formed from copper, and the insulating layers are typically formed from a resinous fiber-impregnated dielectric material. These respective layers can have a wide variety of thickness. For example, they can be on the order of only microns thick, or much thicker.
In manufacturing MLBs, it is desirable to enhance the adhesion between the conducting and insulating layers to avoid delamination in subsequent manufacturing operations or in service. So called “black oxide” processes had been used for years which created a strongly adherent copper oxide layer to which an insulating layer would adhere better. Black oxide processes have, for most of the industry, been replaced by processes such as described in U.S. Pat. No. 5,800,859 involving formation of an organometallic conversion coating (OMCC). These organometallic conversion coating processes involve exposing the copper circuit layer to an adhesion promotion solution, which contains various components including an oxidizer, an inhibitor, and a mineral acid.
One limitation on organometallic conversion coating processes has been that the organometallic conversion coating must be a uniform color, such as, for example, a dark brown or chocolate color. The industry associates this color with a uniform coating which has strong adhesion properties. A dark uniform color is preferred because it provides color contrast with copper to aid in inspection for defects. For example, it provides contrast for inspection for the so-called “pink-ring” defect. Organometallic conversion coating processes which produce significantly lighter coatings are generally unacceptable, or at least undesirable for most applications. For a lighter coating, “pink ring” defects are substantially more difficult to detect.